The status of cyclin D1 and glycogen synthase kinase 3beta (GSK-3beta) was investigated in 41 patients with T1 and T2 squamous cell carcinomas (SCCs) of the tongue. Out of the 41 SCCs, 27 (65.9%) showed overexpression of cyclin D1 in comparison with normal lingual epithelia by an immunohistochemical method. Cyclin D1 gene amplification was detected in only two (9.1%) of 22 informative cases of the SCCs by differential PCR. Expression of GSK-3beta, which was found to regulate proteosomal degradation of cyclin D1 protein, was reduced in 16 cases (39.0%) of the SCCs relative to normal epithelia, and the intensity of GSK-3beta staining showed an inverse association with cyclin D1. These findings suggest that overexpression of cyclin D1 primarily results from stabilization due to reduction of GSK-3beta, but not cyclin D1 gene amplification, in lingual SCCs. Kaplan-Meier analysis demonstrated that the patients with high cyclin D1 and reduced GSK-3beta expression had a significantly lower 5-year survival than the patients with low cyclin D1 and non-reduced GSK-3beta expression (P=0.014). The cyclin D1 and GSK-3beta coupled assessment was more valuable for the prediction of prognosis than assessment based on cyclin D1.